The observation that cancer stem cells possess functional and molecular features found in their normal tissue counterparts has raised the possibility that normal stem cells are preferential and/or requisite targets for oncogenic transformation whose identification could lead to novel strategies for the prevention and treatment of cancer. Trop2 is an epithelial-restricted transmembrane protein that is overexpressed in numerous carcinomas, has oncogenic activity, and is required for colon cancer tumorigenesis. In both the human and mouse prostate, high Trop2 expression marks a sub-population of basal stem cells. Recent experiments have revealed that only the Trop2hi population of human prostate cells is competent to undergo efficient transformation in response to PI3 kinase pathway activation. Interestingly, the Trop2hi cell population is dramatically expanded in histologically normal prostatic tissue prior to the onset of tumorigenesis in the PTEN null mouse model, and the cancers that arise in these mice also express high levels of Trop2. Collectively, these data suggest that Trop2hi cells are targets for transformation and thus Trop2 is likely to play an important functional role in prostate stem cell and tumor biology. However, the extent to which it is actually functionally required in vivo for these processes is unknown in part because of the lack of an appropriate animal model with which to study its role. My laboratory became interested in the function of Trop2 in cancer after we identified an essential role for this protein in colon cancer; therefore understanding its role in tumor biology s one of our long-term goals. The objective of this exploratory R21 application is to determine the extent to which Trop2 is required for prostate stem cell function and tumorigenesis in vivo. The central hypotheses of this proposal are: (1) that Trop2 is required for stem cell function in the prostate: and (2) that Trop2 deletion will suppress prostate tumorigenesis in vivo. To test these hypotheses, we have developed the first Trop2 knockout mouse strain and we propose two specific aims. (1) To evaluate the role of Trop2 in prostate stem cell function. We will measure the regenerative potential of Trop2 null prostate tissue and in vivo and ex vivo and analyze its structure after castration and testosterone-induced regrowth. Since the original submission, we have generated preliminary data revealing a prostate stem cell defect in Trop2-/- cells. (2) To determine the role of Trop2 in PTEN-loss mediated prostate tumorigenesis and metastasis in the mouse. Trop2- and PTEN-deficient mouse strain will be used to evaluate the effects of Trop2 loss of function on the kinetics of tumor development and metastasis in the prostate. We predict that Trop2 deletion will suppress the normal development of PTEN-loss induced prostate tumorigenesis in mice. The identification of molecular components common to the cellular targets of transformation and the tumors derived thereof is expected to produce insights into the requirements for cancer initiation and maintenance. We hope to ultimately apply this information to the development of novels strategies for early detection and treatment.